What Causes Turbo Flutter?by Alexander Eliot
Air pressure that moves backward through the turbocharger and is expelled through the turbo inlet causes a condition commonly known as turbo flutter. The term refers to the noise produced by the turbocharger in this situation, which is due to the backward-moving air disrupting the rotation of the intake turbine. This is caused by various issues with the turbo system.
Turbo flutter, also known as compressor surge, occurs when there is an abundance of pressurized air in the turbo system. When the engine is not able to combust the full amount of air pressurized by the turbocharger, this excess air will build up in the intercooler system. When the pressurization of the air in the intercooler system is greater than the boost value produced by the turbocharger, the air will force its way back out of the system through the turbocharger inlet. Since the intake turbine is flowing air into the system, the outgoing air creates resistance and can damage the turbine.
Turbo surge occurs when the size of the turbocharger is too large to sustain low boost values when the engine is running at low rpm. When the turbocharger begins to spool, the engine is not able to take in the amount of air that is pressurized. The excess air will therefore build up until it is forced back through the turbocharger inlet. In contrast, smaller turbochargers are better suited to maintain a steady flow of low boost, making them less likely to experience turbo flutter.
To find out the boost settings at which your turbocharger will experience turbo flutter, analyze its compressor map. Compressor maps graph a turbocharger's performance, relating its relative atmospheric pressure value to the airflow rate through the turbine. The left side of the compressor map is known as the surge line, which defines the minimum ratio of pressure value and flow rate required to avoid compressor surge. All turbocharger settings that fall to the left of the surge line will create turbo flutter.
One way turbo flutter is prevented is through the use of a blowoff valve. Sometimes called an anti-surge or diverter valve, this device is installed into the intercooler piping between the turbocharger outlet and engine throttle body. A vacuum line connects the intake manifold to a spring-loaded piston device inside the blowoff valve, which keeps the valve closed. When the pressure differential between the intake manifold and intercooler piping reaches a certain level, the blowoff valve spring is depressed. This allows the excess air to be released from the system, preventing it from flowing backward to the compressor.